1. Field of the Invention;
The present invention relates to a tuning control apparatus for a digital tuner. More specifically, the present invention relates to a control apparatus for determining a tuning voltage being applied to a voltage controlled reactance device as a tuning element employed in a digital automatic scanning tuner or a digital preset tuner.
2. Description of the Prior Art;
With the latest developments of large scale integration circuit technology and in view of a recent trend of a digital display of the frequency of a broadcasting signal received by radio receivers, so-called digital tuners have been widely used. Such digital tuners may be classified as an automatic scanning tuner, as disclosed in U.S. Pat. No. 4,085,371, for example, and a preset tuner, as disclosed in U.S. Pat. No. 3,968,440. Taking an example of such automatic scanning tuners upon depression of an upward scanning switch or a downward scanning switch, an automatic scanning operation is initiated, which is achieved by an up/down counter adapted to be stepped responsive to clock pulses, a sequential change of the count value in the counter being converted into an analog voltage, which is applied as a tuning voltage to a voltage controlled variable capacitance diode employed as a tuning element in the tuner. The scanning operation is terminated when the tuner is tuned to the frequency of a broadcasting signal being recieved and the same is detected, whereupon the tuning voltage is controlled such that the tuning frequency is maintained by the tuner. Taking an example of a preset tuner, a plurality of digital data signals associated with the frequencies of the broadcasting signals being selected by the tuner are in advance stored in a memory and one digital data signal corresponding to the frequency of a desired broadcasting signal is selectively read out responsive to operation of a channel selection switch, whereupon the digital data signal is converted into an analog signal, which is then applied as a tuning voltage to a voltage controlled variable capacitance diode employed as a tuning element in the tuner, thereby to achieve reception of the selected broadcasting signal or channel. It is pointed out that the present invention is associated with both the above described digital automatic scanning tuner and the digital preset tuner.
In case of the above described digital automatic scanning tuner, for example, the tuning voltage is continually changed, whereby the correct tuning point or the correct tuning state to the frequency being received by the tuner is detected. However, it could happen that a noise component, an image signal, a spurious signal and the like be detected as a signal being received. As a result, even if such a noise component, a pseudo signal or the like is detected, the automatic scanning operation is terminated in a conventional tuner. On the other hand, a conventional preset tuner also involves such problems as discussed in the following. More specifically, even if a complete tuning state had been established or a broadcasting signal of a given station had been received at the correct tuning point on the occasion of the preset mode and the digital data being stored in a memory had been correctly associated with the frequency of the said broadcasting station, it could happen that by virtue of a temperature variation of the receiver, a time dependent characteristic variation of the circuit components and the like, the digital data as stored in the memory comes not to be associated with the complete tuning point or the correct tuning point of the corresponding broadcasting station on the occasion of a reception mode. Furthermore, even if the presetting was complete and the radio receiver had the same characteristic as that on the occasion of the presetting operation, a possible change of a broadcasting wave depending on the location of reception, such as a "valley" between tall buildings, inside a tunnel, and the like, could cause an obstacle to a complete reception state.